First I want to correct a misconception:

where they forcibly converted or killed any non Muslims, eradicated several religions or religious sects.

The initial expansion of Muslim armies in the 7th century was indeed a military conquest--but it was in no way a circum-Mediterranean campaign of forcible conversion. Indeed, scholars usually talk about Arabization rather than Islamization. The Arab armies were probably not composed entirely of Muslims, first of all, and their most immediate impact on the territories they conquered was Arabic language and involvement in the economic community of the Arab world.

Islamic law, as laid out already in the Quran, establishes Jews and Christians as dhimmis or people of the book. It specifically allows Jews and Christians to practice their religion as long as they pay an additional tax (jizya). We know in practice in the early medieval world, a couple of things ended up happening with that.

First, there were additional restrictions besides simply paying the tax, such as not being able to build new houses of worship (but older ones were preserved), and, of course, not being able to proselytize (convert new believers to Christianity). The extent of additional restrictions as well as their enforcement varied radically from place to place. And in fact, evidence suggests that under the early Rashidun and Umayyad caliphates in the east, even the jizya was not perceived as all that much of a burden. This would change later, especially in the Crusades era: from the 11th-13th century, we have a running list of Egyptian-Jewish complaints about the jizya, along with Arabic sources suggesting that the means of collecting the jizya should involve ritual humiliation. But this is centuries later than the initial conquest.

Second, although the Quran only protects Jews and Christians thanks to the shared Abrahamic heritage, in practice the protection typically extended to other religions as well! In particular, the Zoroastrians in Persia gained some degree of protection. A shade of prudence is still necessary here: the Muslim conquerors were quite clear that the Zoroastrian leadership of Persia needed to go. But this was political expediency, not religious hatred. In smaller cities and in the countryside, Zoroastrians seem to have been allowed to continue practicing their religion.

The important thing to keep in mind in all of this, is that the jizya--applied properly--could be a financial windfall for Muslim rulers. Indeed, in al-Andalus (Islamic Iberia), the Umayyads actively discouraged the lower Christian aristocracy from converting to Islam. They valued the extra income too highly!

As far as signs of outright forced conversion campaigns in the early period (7th-10th centuries): we see a couple of attempts in the modern-day Syria/Palestine region. The most notorious one involves Jews being rounded up and starved in prisons unless/until they convert. HOWEVER. These campaigns were carried out by rogue rebel groups, who (for multiple reasons, not just their treatment of Jews) were squelched by regional governors.

The phrase "conversion or death," however, won't enter the Islamic lexicon until the Almohads in 12th century Spain! (A development that occurs hand-in-hand with rising intolerance in Christendom, by the way, and that the Almohads applied to less-pious Muslims as well as Jews.)

Of course, over time, the carrot and the stick did work nicely to cause people to choose to convert to Islam. I mentioned above how, at certain times and places, the jizya was perceived as a hardship. We know that Jewish and Christian communities developed communal charity to make sure their members could pay the jizya (and in many cases, it would have been a communal rather than individual tax anyway). But even then, you can see how someone might decide it was better to just be Muslim. And there were other benefits, too--access to the Islamic trade network, for example. Richard Bulliett has argued that conversions would have peaked around the 9th-10th century, which actually corresponds nicely with when we see complaints about the jizya.

Unfortunately, we don't have written sources concerning the initial Arab/Muslim expansion. So it's hard to say whether they were motivated by a need to make a "world of submission to God", or whether it was political territorial expansion. Probably there were aspects of both. But the probable presence of non-Muslims in the Arab armies^1 as well as the comfort with maintaining the lower Christian aristocracy in Iberia (though certainly the royalty had to go, can't have fears of revolt) suggests something fundamentally different than the Crusaders, who beat their heads against the wall determined to "rescue" one specific plot of land (Jerusalem) for Christendom.

^1 I differentiate here from making military alliances across religious lines, which was quite common in both Iberia and the Near East; and also from hiring outside mercenaries. One really interesting pattern we see sometimes in the Middle Ages is an ability to work with outsiders who are of a different religion, while being extra-intolerant towards their coreligionists who are native to the kingdom in question.

With the Muslim conquests beginning in the seventh century and carrying well into the high middle ages, where they forcibly converted or killed any non Muslims, eradicated several religions or religious sects.

Can you open this up a little bit? Also how do you explain the existence of non-Muslims today in the areas that were at one point conquered by the Muslims, like Middle East for example or the Balkans or the Iberian Peninsula?